1) Field of the Invention
Processes for preparing tin tetrachloride from the elements, tin and chlorine, have long been known, from EP-B-0 057 929, EP-B-0 094 003 and the literature cited therein, for example. In these processes, tin tetrachloride is circulated and chlorine is passed into the reaction chamber from below. The chlorine dissolved in the tin tetrachloride then enters into reaction with the metallic tin.
2) Description of the Related Art
These processes are well established in practice. However, it has been found that, with high specific conversion rates and/or in start-up operation, the reaction cannot be carried out with sufficient reliability. Disruptions may occur as a result of local overheating, leading to melting of the tin granules. This disrupts the required uniform flow traversal of the tin granules. The consequence may be incomplete chlorine consumption in the reactor, and hence a product of increased chlorine content. This may to a greater or lesser extent restrict the production capacity. In the worst-case scenario, there may be total failure of the plant and, as a result of damage to the reactor, the emergence of chlorine and tin tetrachloride.
EP-B2-0 441 931 discloses a process for continuous preparation of tin tetrachloride by reacting excess metallic tin with chlorine in a reaction chamber in the presence of liquid tin tetrachloride, in which tin tetrachloride is removed in a circuit from the reaction chamber, and is passed back with or without cooling, and the amount of tin tetrachloride removed from the circuit is such that the level of tin tetrachloride in the reaction chamber remains the same, said process comprising passing the chlorine into the circulated stream of tin tetrachloride outside the overall circulating reaction chamber in an amount such that it is dissolved completely in the tin tetrachloride. In this process it is said that the good solubility of chlorine in the tin tetrachloride can be utilized with advantage in order to bring about a favorable reaction regime and to allow the reaction conditions to be easily adjusted. When liquid chlorine is used it is said to be possible judiciously to take the heat of evaporation required from the heat of reaction present in the liquid tin tetrachloride emerging from the reactor.
In addition, a slight temperature difference is indicated between the entry of the tin tetrachloride into the reaction chamber (70 to 80.degree. C.) and the exit from the reaction zone (91.degree. C., or 10.degree. C. difference between entry and exit of the main reaction zone of the tin bed). However, this known process gives relatively low space-time yields.
Depending on the space-time yield, in the upper limiting range the difficulties specified at the outset may nevertheless still occur. Although it is in accordance with the teaching of process engineering to counter reactor overheating by temperature monitoring and automatic chlorine shutoff, local overheating of the reaction zone still always puts at risk or limits the plant capacity or availability. Another critical safety point is the loss of the tin tetrachloride supply stream, which is also necessary for dissipating the heat of reaction. In this case the automatic shutoff of the chlorine supply in accordance with the teaching of process engineering is an absolute safety necessity in order to rule out an uncontrolled reaction regime as a result of oversupply of chlorine, with the danger of plant destruction. Nevertheless, the restarting of the plant remains a critical step, which can very easily proceed in an uncontrolled manner. The moment of risk is determined by disruption of the tin bed in the reaction zone in the case of sudden pressure drop, as is the case with failure of the tin tetrachloride circulation pump.
The fine to ultrafine tin sludge formed by reaction of the tin granules in the reaction zone, which has a greatly increased tin surface area, can react with excess chlorine and so give rise to critical temperature peaks. Important in this context is the strong temperature-dependency of the solubility of chlorine in tin tetrachloride; the temperature of the tin tetrachloride circuit which is normally low under startup conditions, corresponds to a chlorine solubility which is much higher than that at the customary reaction temperature. For example, at 20.degree. C. 125.5 g of chlorine dissolve in 1 l of tin tetrachloride, and at 30.degree. C. a further 96 g of chlorine, in each case at 1 bar of chlorine pressure.